1. Field of the Invention
Without limiting its scope, this invention relates to rapid prototyping, and more particularly to a system, method, and process for manufacture of three-dimensional objects from computer data using computer-controlled dispensing of multiple media and selective material subtraction.
2. Description of the Related Art
As complex designs increase the need for rapid prototype fabrication, this need for immediate feedback requires model or machine shops to fabricate complex parts in low volume with minimum setup and run-time. Most fabrication methods, however, are slow, complex, and expensive.
While manual machining and forming methods are often cheap and effective for simple designs, the costs can be prohibitive for the iterations required of complex parts and assemblies. Computer Numerically Controlled (CNC) machines are widely used to automate complex fabrication, but are costly to operate, maintain, and program just for one-of-a-kind production.
The most widely known system in the field of rapid prototyping is stereolithography. This system fabricates complex parts from computer data by employing a set of computer-controlled mirrors to scan a laser beam across selected two-dimensional areas of liquid photopolymer contained in a vat and thereby form a layer of solid polymer. The cured layer, which is attached to a platform, is lowered into the vat and new layers are generated one on top of the previous layers to form a three-dimensional part.
When the part is complete, the excess resin is removed with a solvent and the platform attachment as well as all overhang supports are cut away from the desired object. Additional light exposure is required to solidify any trapped liquid.
A major drawback to stereolithography and similar approaches is that support structures must be designed to join the object to the platform and attach any overhangs, large spans or disjoint areas. The addition of these structures to the CAD model and subsequent manual removal from the part during cleaning is labor intensive and often requires special skills.
Another drawback is the additional occupational and environmental safety measures required with the use of lasers or resins. The chemicals used in this process and in cleanup require special handling, ventilation, and storage to protect the operator and the work place. High volumes of waste are generated in resin removal and cleanup. The photopolymer is expensive and nonrecyclable. All of this makes installation in common work areas or offices impractical for size and environmental reasons. Furthermore, because of the delicate nature of lasers and optics, installation and calibration is very difficult. Maintenance is expensive due to system complexity and laser costs.
Another lithographic fabrication method is selective laser sintering. This method employs a heat laser to fuse (sinter) selected areas of powdered material such as wax, plastic, or metal. In practice, a vat of powder is scanned by the laser thereby melting individual particles which then stick to adjacent particles. Layers of the fused powder are processed sequentially like photopolymer lithography. An advantage of the sintering method is that the non-heated powder serves as a support for the part as it is formed. This means that the non-heated powder can be shaken or dusted off the object.
Selective laser sintering, however, is also a complex, expensive optical system. The resolution of the final part is limited by the beam diameter, which is typically 0.01"-0.02". Furthermore, in an additional step, the powder is deposited and levelled by a rolling brush which requires other electro-mechanical components. Unfortunately, levelling fine powders with a rolling brush often causes nonhomogeneous packing density. Additionally, while power costs less (material & labor) than liquid photopolymer systems, preparing a 30 micron layer is difficult. An object built from this powder is of medium resolution, has a non-uniform surface and, often, a non-homogeneous structure.
Research has been conducted at the Massachusetts Institute of Technology in fabrication by three-dimensional printing. In this research, ceramic powder is deposited using a wide feeder over a vat or tray. A silica binder is then printed on selected areas of the powder to form a solid cross-section. The process is repeated to form a stack of cross-sections representing the final object.
This approach exhibits the same powder deposition problems as selective laser sintering, along with the additional difficulty in removing unbound powder from internal cavities. Furthermore, objects generated by this system are not recyclable. The MIT research is directed toward the production of ceramic molds. Metal or other materials are then injected or poured into the mold which is later broken away from the cast parts. Unfortunately, the mold's internal cavities, which define the final parts, are not easily inspected, which leads to an expensive trial and error process to acquire accurate parts.
Additional problems found with the art have been an inability to: provide for variable surface color or use more than one material media in the fabrication of the desired object; remove the media support for overhangs, large spans or disjoint areas automatically; or provide an automated system for physically reproducing three-dimensional computer designs and images. Systems currently available are expensive, the media they use cannot be recycled, and they cannot provide for automated part handling after fabrication due to their use of bulk powders and resins, which require containers rather than conveyor platforms. Accordingly, improvements which overcome any or all of these problems are presently desirable.
By way of further background, U.S. Pat. No. 4,665,492, issued May 12, 1987 to Masters, describes a computer-automated process and system for fabricating a three-dimensional object. The disclosed method requires the use of an origination seed at which particles of the part-forming material are directed, and to which the particles adhere to form the object. As such, it is believed that the complexity of the shape of objects formed by the disclosed method is limited, as only those objects which can be formed in a unitary manner from the origination seed can be produced by this method.
By way of still further background, U.S. Pat. No. 4,961,154 issued Oct. 2, 1990, and European Patent Office Publication No. 0 322 257 published Jun. 6, 1989, disclose methods for producing three-dimensional parts by the selective photoexposure of photopolymers. Each of these references disclose a method and apparatus by which a layer of photopolymer is dispensed and selectively exposed to light, followed by development of the exposed layer. These references further disclose that a different support material may be substituted for the non-polymerized photopolymer in each layer by removing the non-polymerized photopolymer, and filling those portions of the layer from which the non-polymerized polymer were removed with a different support material, after which the next object layer is formed in a similar manner. The methods disclosed in these references are limited to photopolymer processing, and as such are useful to produce a part from only a narrow set of materials. In addition, the machines for producing a part according to this method are necessarily quite complex, considering that non-polymerized material must be removed and disposed of during the processing of each layer, requiring the transport of the material being processed from station to station in the machine.